An organic electroluminescent device (hereinafter, electroluminescent is occasionally abbreviated as EL) is a self-emitting device based on the principle that, when an electrical field is applied, a material emits light using energy generated by a recombination of holes injected from an anode with electrons injected from a cathode.
A typical emitting material used for an organic EL device is a fluorescent material that emits fluorescent light by a singlet exciton. However, recently, it is suggested to use a phosphorescent material that emits phosphorescent light by a triplet exciton instead of the fluorescent material (see, for instance, non-Patent Documents 1 and 2).
When electrons and holes are recombined in an organic EL device, it is presumed that a singlet exciton and a triplet exciton are produced at a rate of 1:3 due to difference in spin multiplicity.
Accordingly, an organic EL device using a phosphorescent material as in non-Patent Documents 1 and 2 can achieve three to four times higher luminous efficiency than an organic EL device that emits light using only fluorescent material.
The phosphorescent material generally exhibits a large excited triplet energy gap (Eg(T)). Accordingly, when the phosphorescent material forms an emitting layer as a dopant, a host material having a larger Eg(T) is used.
Since the host material having a large Eg(T) has a high affinity level (Af), an electron injected from the cathode to the emitting layer is not recombined with a hole in the emitting layer and is likely to be transferred into the anode.
A known method to solve such a problem is to form a hole transporting layer by using a material having a higher Af than the host material of the emitting layer and trapping electrons in the emitting layer.
With this arrangement, electron blocking of the hole transporting layer can enhance probability of recombination of charges in the emitting layer, thereby providing phosphorescent emission with high efficiency.
When the triplet energy gap (Eg(T)) of the hole transporting layer is large, inactivation caused by excited triplet generated in the emitting layer being transferred into the hole transporting layer is unlikely to occur, thereby providing an efficient phosphorescent emission.
However, on electron blocking, the electrons concentrate on an interface between the emitting layer and the hole transporting layer. The concentration of the electron may promote degradation of the materials and reduce lifetime of the device. Accordingly, the hole transporting layer needs to be highly tolerant of the electrons.
Further, as the name implies, the hole transporting layer needs to have capability for transporting holes.
In short, the hole transporting layer of the phosphorescent organic EL device needs to have all of electron blocking capability, electron tolerance and hole transporting capability.
As such a material, tris-4,4′,4″-carbazoyl-triphenylamine (TCTA) described in non-Patent Document 3 is typically used, which has not provided sufficient lifetime to the device.
[non-Patent Document 1] Applied Physics letters Vol. 74 No. 3, pp 442-444
[non-Patent Document 2] Applied Physics letters Vol. 75 No. 1, pp 4-6
[non-Patent Document 3] Angewandte Chemie International Edition Vol. 46, 2418-2421